1. Field of the Invention
This invention relates generally to transverse flux electrical machines. The present invention more specifically relates to a core for transverse flux alternators and assembly thereof.
2. Description of the Related Art
Alternators and motors are used in a variety of machines and apparatuses to produce electricity from mechanical movements. They find applications for energy production and transportation, to name a few. Alternators and motors can use Transverse Flux Permanent Magnet (TFPM) technologies.
Transverse flux machines with permanent magnet excitation are known from the literature, such as the dissertation by Michael Bork, Entwicklung und Optimierung einer fertigungsgerechten Transversalfluβmaschine [Developing and Optimizing a Transverse Flux Machine to Meet Production Requirements], Dissertation 82, RWTH Aachen, Shaker Verlag Aachen, Germany, 1997, pages 8 if. The circularly wound stator winding is surrounded by U-shaped soft iron cores (yokes), which are disposed in the direction of rotation at the spacing of twice the pole pitch. The open ends of these U-shaped cores are aimed at an air gap between the stator and rotor and form the poles of the stator. Facing them, permanent magnets and concentrators are disposed in such a way that the magnets and concentrators that face the poles of a stator core have the opposite polarity. To short-circuit the permanent magnets, which in the rotor rotation are intermittently located between the poles of the stator and have no ferromagnetic short circuit, short-circuit elements are disposed in the stator.
Put otherwise, transverse flux electrical machines include a circular stator and a circular rotor, which are separated by an air space called air gap, that allows a free rotation of the rotor with respect to the stator, and wherein the stator comprises soft iron cores, that direct the magnetic flux in a direction that is mainly perpendicular to the direction of rotation of the rotor. The stator of transverse flux electrical machines also comprises electrical conductors, defining a toroid coil, which is coiled in a direction that is parallel to the direction of rotation of the machine. In this type of machine, the rotor comprises a plurality of identical permanent magnet parts, which are disposed so as to create an alternated magnetic flux in the direction of the air gap. This magnetic flux goes through the air gap with a radial orientation and penetrates the soft iron cores of the stator, which directs this magnetic flux around the electrical conductors.
In the transverse flux electrical machine of the type comprising a rotor, which is made of a plurality of identical permanent magnet parts, and of magnetic flux concentrators, the permanent magnets are oriented in such a manner that their magnetization direction is parallel to the direction of rotation of the rotor. Magnetic flux concentrators are inserted between the permanent magnets and redirect the magnetic flux produced by the permanent magnets, radially towards the air gap.
The transverse flux electrical machine includes a stator, which comprises horseshoe-shaped like soft iron cores, which are oriented in such a manner that the magnetic flux that circulates inside these cores, is directed in a direction that is mainly perpendicular to the axis of rotation of the rotor.
The perpendicular orientation of the magnetic flux in the cores of the stator, with respect to the rotation direction, gives to transverse flux electrical machines a high ratio of mechanical torque per weight unit of the electrical machine. Eddy currents influence the magnetic efficiency.
Eddy currents (also called Foucault currents) are circular electric currents induced within conductors by a changing magnetic field in the conductor, due to Faraday's law of induction. Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field. They can be induced within nearby stationary conductors by a time-varying magnetic field created by an AC electromagnet or transformer, for example, or by relative motion between a magnet and a nearby conductor. The magnitude of the current in a given loop is proportional to the strength of the magnetic field, the area of the loop, and the rate of change of flux, and inversely proportional to the resistivity of the material.
By Lenz law, an eddy current creates a magnetic field that opposes the magnetic field that created it, and thus eddy currents react back on the source of the magnetic field. For example, a nearby conductive surface will exert a drag force on a moving magnet that opposes its motion, due to eddy currents induced in the surface by the moving magnetic field. This effect is employed in eddy current brakes, which are used to stop rotating power tools quickly when they are turned off. The current flowing through the resistance of the conductor also dissipates energy as heat in the material hence having an adverse effect on electrical machines efficiency. Thus eddy currents are a source of energy loss in alternating current (AC) inductors, transformers, electric motors and generators, and other AC machinery, requiring special construction such as laminated magnetic cores to minimize them.
Cores made of a stack of sheet material radially laminated and angularly stacked along the coil of the TFEM is channeling the flux therein while producing circular eddy currents in the lamination plane that are not restrained in the thickness of the lamination. The purpose of stacking laminated sheet material is to decrease the eddy current losses, which is not the case when the motor is in the unaligned positon. The coil needs to be more massive to compensate the lower global efficiency of the TFEM by reducing the Joules losses (conducting losses). The cores housing, that is not laminated, is also more complex to manufacture and assemble to hold each core stack together during the assembly of the stator and part of the magnetic flux is loss to the housing when the magnetic concentrators are in the unaligned position. Other detrimental issues are occurring when honing the stator's interior like a separation of the laminated sheets cores.
It is therefore desirable to provide a core design that is minimizing eddy currents. It is desirable to produce a core for an electrical machine that is easy to assemble. It is also desirable to provide a core for an electrical machine that is economical to produce. Other deficiencies will become apparent to one skilled in the art to which the invention pertains in view of the following summary and detailed description with its appended figures.